This invention relates generally to the art of guidance systems for airborne vehicles, and more specifically concerns a guidance system in which the source of signal illumination is separate from the receiver, such as on a satellite.
In certain circumstances, it is important that opportunities for the discovery of an airborne vehicle, such as a missile, be minimized. Unfortunately, conventional missiles do provide many such opportunities for detection. For example, the guidance systems of most aircraft, including missiles, utilize radar emissions. Such guidance systems usually have both an altitude determining capability and a ground map capability. Ground mapping generally refers to a system of terrain correlation, in which the information produced by the on board guidance system concerning the terrain configuration beneath the airborne vehicle is compared with an actual map of the terrain, which has been previously stored in memory. The emissions of such a guidance system, however, identify the presence of the missile; they in effect act as a beacon, leading to the possible detection of the airborne vehicle and the opportunity for counter measures against the vehicle.
There have been several attempts to minimize the possibility of detection. In one approach, the emissions of the guidance system of a conventional missile are disguised to appear to be random noise. One example of this approach is commonly known in the art as a low probability of intercept radar (LPIR). The emissions of an LPIR system are still detectable, however, and if correctly interpreted, also present a possibility of detection. In another approach, the guidance system is emissionless, i.e. it is a passive guidance system. An example is a radiometric frequency guidance system which uses radiometric frequency energy generated by objects on the ground to provide the required information from which the location of the aircraft may be determined. The radiometric frequency range used is typically from 30 gigahertz to 300 gigahertz. Radiometric systems, however, are not completely satisfactory, since radiometric signals are attenuated by weather, and their image accuracy is relatively poor. Furthermore, the information from a radiometric guidance system is two-dimensional, since range data is not obtainable with passive radiometric systems. Finally, since point sources of radiometric energy become elongated as the range increases in the forward direction, the resolution of radiometric systems, aside from the area directly beneath the aircraft, is relatively poor.
Thus, there is currently a significant need for guidance systems for airborne vehicles such as missiles, which are essentially passive in operation in that they produce virtually no detectable emissions.
Accordingly, one object of the present invention is to provide a guidance system for aircraft and the like which does not utilize emissions from the aircraft.
Another object of the present invention is to provide such a system in which the source of emissions is based on a satellite.
It is a further object of the present invention to provide such a system in which the emissions are disguised so that they do not look like radar emissions.
It is a further object of the present invention to provide such a system which is capable of producing both ground map and altitude information in all weather conditions.